This invention relates to the treatment of gases. In particular, it relates to treatment of a gas stream comprising hydrogen sulphide.
Gas streams comprising hydrogen sulphide are typically produced as waste products or by-products in many industrial processes. For example, acid gas streams comprising carbon dioxide and hydrogen sulphide are typically produced during oil refinery operations in which sulphur is removed from crude oil. It is necessary to treat such hydrogen sulphide containing streams before discharging them to the atmosphere so as to reduce or remove altogether their content of sulphur-containing gases. One well known, widely practised process for the treating of gas stream comprising hydrogen sulphide is the Claus process. This process is based on the reaction between hydrogen sulphide and sulphur dioxide to form sulphur vapour and water vapour in accordance with the equation: EQU SO.sub.2 +2H.sub.2 S.dbd.2H.sub.2 O+3S
Sulphur exists in the vapour phase in a number of different molecular species such as S.sub.2, S.sub.6 and S.sub.8 according to the temperature.
The first stage of the Claus process is to burn approximately a third of the hydrogen sulphide in the incoming gas stream to form sulphur dioxide and water vapour in accordance with the equation: EQU 2H.sub.2 S+3O.sub.2 .dbd.2H.sub.2 O+2SO.sub.2
This combustion reaction takes place in a suitable furnace and normally air is used as a source of oxygen for the purposes of combustion. Reaction between the sulphur dioxide and hydrogen sulphide starts in the combustion zone and then continues downstream of the combustion zone. It is, however, a feature of the Claus reaction that at the temperature that is created by the combustion of hydrogen sulphide, it is not possible (with air) to convert more than about 75% of the remaining hydrogen sulphide to sulphur by reaction with sulphur dioxide, and typically between 50 to 70% of the hydrogen sulphide is so converted. It is, however, possible to achieve a higher total conversion in the presence of a catalyst at a reaction temperature in the order of 200.degree. to 450.degree. C. by reacting the remaining hydrogen sulphide and sulphur dioxide. Accordingly, after the gases pass out of the furnace they are cooled to a temperature at which the sulphur that is formed in the furnace condenses. The sulphur is thus recovered. The gases are then reheated to a temperature suitable for the performance of a catalysed reaction between hydrogen sulphide and sulphur dioxide, such temperature typically being in the order of 200.degree. C. Typically, two or three stages of catalytic conversion are performed, with the hydrogen sulphide containing gas stream being reheated immediately upstream of each stage and resulting sulphur being separated from the gas stream by condensation immediately downstream of each stage. The resulting gas mixture now containing only a relatively low concentration of sulphur-containing gases is then typically passed to a tail gas clean-up process or is incinerated. Suitable tail gas clean-up processes include the Scot, Beavon and Stretford processes.
In order to improve the conventional Claus process, it is now well known to use pure oxygen or oxygen-enriched air instead of air unenriched in oxygen to support combustion of the hydrogen sulphide. This substitution reduces the proportion of nitrogen in the gas stream that flows through the Claus plant and accordingly enables a plant of given size to be uprated. In practice, however, in many plants, the amount of uprating that can be achieved by this method is limited as there is a tendency for the reduced volume of nitrogen to lead to higher temperatures within the furnace that cannot be withstood by the waste heat boiler associated with the furnace or by the refractory lining of the furnace. Indeed, the more concentrated in hydrogen sulphide the gas stream, the less becomes the amount of uprating can be achieved by simple substitution of oxygen for air.
There have therefore been a number of proposals in the art to tackle the problem of excessive temperature rise that can be caused by the substitution of oxygen for air. In EP-A-0 165 609 it is disclosed that enriching the combustion air with oxygen to a level of 70 mole percent oxygen produces a calculated theoretical adiabatic flame temperature of about 3750.degree. F. (2065.degree. C.), but that by recycling part of the gas stream leaving the first sulphur condenser (which is intermediate the furnace and the first catalytic stage) to the furnace itself so as to moderate the flame temperature, this temperature can be kept to below 2800.degree. F. (1538.degree. C.) while achieving an increase in throughput of hydrogen sulphide in the range of 50 to 100% by volume. This result can be achieved since the recycle stream consists largely of water vapour (steam) which has a higher molar heat capacity than nitrogen. A number of alternative methods of moderating the flame temperature have been proposed. For example, GB-A-2 173 780 proposes that the temperature be moderated simply by introducing liquid water into the flame zone. In EP-A-0-252-497 it is proposed to use a temperature moderating stream of sulphur dioxide. The sulphur dioxide may be imported or generated by burning a small fraction of hydrogen sulphide feed or liquid sulphur product in a separate process unit. Alternatively, it can be generated from a `back end` Claus process stream (which generally contains about 3 moles per 100 moles of hydrogen sulphide feed). Additional advantages that can be obtained from this method are reduced oxygen consumption, increased percentage conversion of hydrogen sulphide and an increase in the capacity of the furnace in which the hydrogen sulphide is burnt.
An alternative approach to using pure oxygen or oxygen-enriched air to improve the capacity or throughput of a Claus process is to conduct the combustion of the hydrogen sulphide in two separate furnaces. Accordingly, the overall amount of heat generated by the combustion is allocated between the two furnaces without the need to employ an external or recycled moderator of temperature. Such a process is described in GB-B-2 187 445. In a variation of this approach, a minor part of the hydrogen sulphide containing feed stream can be fully combusted in a first furnace using substantially pure oxygen to support the combustion and a recycle stream of sulphur dioxide and water vapour to moderate the temperature in the first furnace. A part of the resulting gas mixture is cooled and introduced into a second or main furnace so as to reduce the amount of sulphur dioxide that needs to be formed therein by the combustion of hydrogen sulphide. Examples of such a process are described in GB-B-2 187 444 and EP-A-0 290 286. Staging the combustion over two furnaces makes it possible to gain a greater increase in capacity and hydrogen sulphide throughput than is typically possible from a process using but a single furnace with introduction of a moderator into the flame zone so as to moderate the temperature of an oxygen-enhanced flame.
The prior processes discussed above all concentrate on the use of pure oxygen or oxygen-enriched air to improve the throughput or capacity of a Claus plant including one or more furnaces and one or more catalytic stages. One of the main contributions to the capital and running costs of a Claus plant is from the catalytic stages. The catalyst is relatively expensive and requires periodic replacement. Moreover, reheat means is required upstream of each stage. A need to reduce the number of catalytic stages employed for a given percentage conversion of the hydrogen sulphide in the feed gas is identified in EP-A-0 328 820. EP-A-0 328 820A discloses using at least three and typically four furnaces to increase the amount of conversion of hydrogen sulphide that takes place upstream of the catalytic stage or stages. Each such furnace employs pure oxygen or oxygen-enriched air to support combustion of hydrogen sulphide. The number of furnaces employed is itself a disadvantage.